1. Field of the Invention
This invention relates to mineral binder systems which contain a carbon black product as a colorant.
2. Discussion of Related Art
Mineral binder systems used to form items such as concrete, cement, mortar and exterior plaster formulations are often colored to enhance their aesthetic appeal. Coloring can be accomplished either by applying a suitable coating to the exposed surfaces or by adding small amounts of one or more pigments to the mineral binder system to uniformly color the mix. Since surface coatings are subject to peeling, fading and weathering, the latter method of coloring is preferred. The pigment or pigments can be added either to the dry mineral mix, for example, in the case of concrete to the cement-sand mixture, or to the water used to set such a mix.
Pigments which are suitable for coloring mineral binder systems which are exposed to outdoor conditions must 1) be alkali-resistance, 2) be lightfast, 3) be resistant to industrial atmospheres, and 4) weather at a comparable rate with the body in which it is mixed so that the appearance of the surface does not change substantially with time.
In addition, for ease of application, the pigment should be relatively dust-free, and should easily disperse in the mix to attain its maximum coloring power.
Black pigments are desirable colorants for use in mineral binder systems because a large variety of colors and color shades can be obtained by their use, either alone or in combination with other pigments. Black iron oxides are the most preferred black pigment, but carbon blacks are also used to a limited extent.
While carbon blacks exhibit excellent coloring properties, alkali-resistance, lightfastness and chemical stability, they are not preferred in mineral binder systems exposed to outdoor weathering. Weathering studies show that the surface appearance of bodies containing carbon black undesirably changes as the weathering process progresses. When the system contains only carbon black as the coloring pigment, the surface fades. When the carbon black is used in combination with other colorants, the appearance of the other colorants become more pronounced. This change in carbon black-pigmented mineral systems has been attributed to the leaching out and washing away of the carbon black pigment particles, which are very small relative to the other ingredients. This has limited its use in systems exposed to outdoor weathering.
Additionally, depending on the physical form in which it is supplied, carbon black can be either very dusty or very difficult to disperse. The process used for incorporating the carbon black into a binder system depends on both the form in which the pigment is supplied and the processing equipment available to the user. As produced, carbon blacks are powdery materials with bulk densities ranging from about 0.02 to 0.1 g/cc and are termed fluffy blacks. Such blacks are very dusty. Because of their low densities and large surface areas, the fluffy products are cohesive, have very poor conveying properties and are therefore difficult to bulk handle. For this reason fluffy products have limited utility, and are generally supplied in bagged form. Fluffy blacks are, however, dispersible and can develop their full coloring potential by relatively simple grinding procedures.
To improve the bulk handling properties of carbon blacks and reduce their dustiness, fluffy blacks are typically densified by various pelletizing procedures to attain bulk densities ranging from about 0.2 to 0.7 g/cc. For a given grade of carbon black, handling properties tend to improve with increasing degrees of densification. Dispersibility, on the other hand, is progressively degraded as the extent of densification is increased. Thus there is a tradeoff between improvement in bulk handling properties and degradation in dispersibility. Because of the advantages of increased cleanliness, however, pelletized carbon blacks are often used for introducing carbon blacks into mineral binder systems. In such cases, however, the extent of grinding or milling required to form a uniform, intimate mix will be greater than that which is employed with the fluffy form of the product.
Carbon black has been added to mineral binder systems in a variety of ways. It can be milled into the dry sand-cement mixture, for example, and then the requisite amount of water necessary for setting the mix can be added. Alternately, an aqueous dispersion of the carbon black, in all or part of the requisite volume of water necessary for setting the mix, can be uniformly blended into the sand-cement mixture. In either case, for full and uniform color development, the carbon black agglomerates must be broken down to yield primarily individual aggregates (the smallest dispersible units of carbon black). This is accomplished either by milling the dry mix or by predispersing, by milling, the carbon black in the aqueous medium. Since carbon blacks tend to be hydrophobic, surface active agents are often used to promote wetting. In addition, the presence of such an agent in the aqueous medium can enhance the dispersion process and aid dispersion stabilization.
Attempts have been made to improve both the weathering and dispersing properties of carbon blacks used to pigment mineral binder systems. For example, U.S. Pat. No. 4,006,031 discloses that fluorine-containing wetting agents used with carbon blacks provides improved weathering properties to mineral binder systems. Others, as described in European Patent No. 50354, have used surface-active polymers which disperse the carbon black in the aqueous medium used to set the mineral binder system and then become inactive after drying. The benefits include better black dispersion, improved weathering resistance and decreased efflorescence. The disclosure of these patents is herein incorporated by reference.
Nevertheless, even in these prior art processes, the carbon black, whether in fluffy or pelletized form, must be milled attain the required degree of pigment dispersion. Thus, there remains a need for carbon blacks useful as colorants in mineral binder systems which can be used in pelletized form, yet easily disperse with low shear stirring and less readily wash out of the system during weathering.